1. Field of the Invention
The present invention relates to a beam shaper, and a semiconductor laser source device and an optical head, both using the beam shaper. More specifically, the present invention relates to a beam shaper that can convert a luminous flux emitted from a semiconductor laser source, whose beam divergence angle is different in the horizontal direction and the vertical direction, into a luminous flux having a smaller difference between the beam divergence angles, and a semiconductor laser source device and an optical head, both using the beam shaper.
2. Related Background Art
The distribution of light quantity of a beam emitted from a semiconductor laser has an elliptical shape. This is because the beam divergence angle is larger in the vertical direction but smaller in the horizontal direction. In a device provided with a semiconductor laser as its light source, for example, in an optical disk device, the spot size of a beam collected by an objective lens should be narrowed to be as small as possible. Therefore, it is preferable that the distribution of light quantity is uniform as much as possible. The distribution of light quantity can be made uniform by using only light located in the vicinity of the center of the semiconductor laser. To do so, however, a large amount of light emitted from the semiconductor laser has to be eliminated, which would deteriorate the efficiency of laser power. This deterioration becomes a significant problem for rewritable optical disk devices.
To cope with the above problem, several methods have been employed to convert the distribution of light quantity having an elliptical shape in a semiconductor laser into a more circular shape. For instance, in known devices, a light beam emitted from a semiconductor laser is converted into a parallel beam with a collimator lens, and then the distribution of light quantity shaped like an ellipse is converted into a circular shape with two assembled prisms. This construction, however, necessitates a space for arranging the prisms subsequent to the collimator lens, which makes the optical system larger. As a solution thereof, JP61(1986)-254915 A, JP1(1989)-109317 A, and JP9(1997)-258099 A each propose an optical device combining the functions of a beam shaper and a collimator lens. These proposed devices, however, have problems in that their aberration correction is insufficient, their positioning tolerance is extremely tight, and also it is difficult to machine such devices because of the complexity of their surface configuration. Further, JP6(1994)-294940 A proposes an optical device having a beam shaping function only, aside from a collimating function. More specifically, this optical device has a cylindrical surface at a side of a semiconductor laser and a toroidal surface located opposite thereto. This construction, however, has a problem in that the aberration changes considerably with respect to displacement in a relative position between the light source and the optical device and therefore a high degree of location accuracy is required.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a beam shaper with a small aberration variation and a high positioning tolerance, and to provide a semiconductor laser source device and an optical head by which the efficiency of laser power can be improved.
To achieve the above object, the beam shaper according to the present invention, which converts a luminous flux emitted from a semiconductor laser source whose beam divergence angle is different in a horizontal direction and a vertical direction into a luminous flux having a smaller difference, where a beam divergence angle in a far field from the semiconductor laser source is larger in the vertical direction and smaller in the horizontal direction, includes a single lens whose both surfaces are toric surfaces. Here, both of the toric surfaces have aspheric surfaces in a cross-section in the vertical direction, both of the toric surfaces have spherical surfaces in a cross-section in the horizontal direction, and both centers of curvature of the spherical surfaces are located substantially at a luminous point of the semiconductor laser source.
With this configuration, a beam shaper whose positioning tolerance is high in both cross-sections in the horizontal direction and the vertical direction, and whose wavefront aberration as a whole is small can be realized. In addition, since both surfaces of the beam shaper in one cross-section are spherical, it becomes relatively easy to machine such a beam shaper, as compared with a beam shaper whose both surfaces in both cross-section are aspheric surfaces.
In the aforementioned configuration of the beam shaper, it is preferable that the toric surfaces in the cross-section in the vertical direction are a convex surface at the side of the semiconductor laser source and a concave surface at the side opposite to the semiconductor laser source. With this preferred example, a beam shaper whose positioning tolerance is high and whose wavefront aberration as a whole is small can be realized.
In addition, in the aforementioned configuration of the beam shaper, it is preferable that the following Formula 1 is satisfied:
[Formula 1]
1.2 less than NAy/NAx less than 3.0 
where NAx and NAy represent numerical apertures, which the beam shaper can take in from the semiconductor laser source, in the cross-sections in the horizontal direction and the vertical direction, respectively. When falling below the lower limit in Formula 1, that is, when the value of NAy/NAx is 1.2 or less, the degree of correction is lowered, which would deteriorate a laser capture efficiency. On the other hand, when exceeding the upper limit in Formula 1, that is, when the value of NAy/NAx is 3.0 or more, a power of the beam shaper in the vertical cross-section becomes too large, which makes the positioning tolerance tighter, and thus leads to a practical problem.
The semiconductor laser source device according to the present invention includes a semiconductor laser source and a beam shaper, where a beam divergence angle in a far field from the semiconductor laser source is larger in a vertical direction and smaller in a horizontal direction, and the beam shaper includes a single lens whose both surfaces are toric surfaces. Here, both of the toric surfaces have aspheric surfaces in a cross-section in the vertical direction, both of the toric surfaces have spherical surfaces in a cross-section in the horizontal direction, and both centers of curvature of the spherical surfaces are located substantially at a luminous point of the semiconductor laser source. With this configuration, a semiconductor laser source device with a high efficiency of laser power can be realized.
In the aforementioned configuration of the semiconductor laser source device, it is preferable that the device further includes a collimator lens that converts a luminous flux emitted from the beam shaper into a parallel beam. With this preferred example, a semiconductor laser source device producing a high-quality parallel beam can be realized. In addition, in this case, it is preferable that an aberration correction for the collimator lens is made when the beam shaper is inserted between the collimator lens and the semiconductor laser source. With this preferred example, a laser beam emitted from the semiconductor laser source device can be converted into a parallel beam without being affected by the beam shaper very much.
In addition, in the aforementioned configuration of the semiconductor laser source device, it is preferable that the toric surfaces in the cross-section in the vertical direction are a convex surface at the side of the semiconductor laser source and a concave surface at the side opposite to the semiconductor laser source.
Further, in the aforementioned configuration of the semiconductor laser source device, preferably, the following Formula 2 is satisfied:
[Formula 2]
1.2 less than NAy/NAx less than 3.0 
where NAx and NAy represent numerical apertures, which the beam shaper can take in from the semiconductor laser source, in the cross-sections in the horizontal direction and the vertical direction, respectively.
In addition, in the aforementioned configuration of the semiconductor laser source device, preferably, the beam shaper is made of resin, and the semiconductor laser source and the beam shaper are fixed with a resin lens barrel. In the case of a beam shaper fabricated by plastic molding, an astigmatism would occur due to temperature change. If the semiconductor laser source and the beam shaper are fixed with a resin lens barrel having an adequate length, generation of the astigmatism can be suppressed.
The optical head according to the present invention includes a semiconductor laser source device, collecting means for collecting light emitted from the semiconductor laser source device onto an information medium, light splitting means for splitting the light modulated on the information medium, and light-receptive means for receiving the light modulated on the information medium. With this configuration, the number of components can be decreased, and a low cost optical head that is easily assembled can be provided. In addition, since the efficiency of laser power emitted from the semiconductor laser can be improved, recording speed can be increased and the power of the semiconductor laser source can be lowered, in particular, in rewritable optical disks.